1. Field of Invention
This invention relates to amplifier circuits, specifically to those with constant current output stages.
2. Description of Prior Art
Class A transistor amplifiers are well known in the art. Traditionally, class A bias is used to eliminate switching distortion and stabilize device parameters against changes in temperature and load impedance. It has been established that variations in supply voltage due to load current degrade the signal quality of such amplifiers. Load currents can temporarily pull the supply voltage down, thereby changing device operating points and causing distortion. In single supply multichannel amplifiers load transients can couple to other channels causing crosstalk. Additionaly, high frequency interstage coupling can cause oscillation due to unpredicted feedback paths.
Output devices and loads are usually protected against overstress caused by a transient signal or load condition. The design goal of such protection is to prevent overstress failures without degrading signal quality during normal operation.
Heretofore, attempts to minimize the effects of supply voltage variations due to load current in transistor audio amplifiers include the use of increased filter capacitance and inductance, additional transformer power, active regulation, and separate supplies for different stages and channels. Prior vacuum tube art includes several constant current audio amplifier topologies. In these non-bridge designs the alternating current drawn from the supply was reduced, but with a loss of efficiency. Vacuum tube bridge topologies were used to eliminate the output transformer. The constant current technique did not survive the transition to solid state amplifier designs where the normally used class AB bias precludes constant current operation.
Methods to protect output devices and loads usually involve active current limiting, relays, or fuses in the signal circuitry. Prior art protection topologies generally have some effect on signal quality. Protection circuitry requirements are more restrictive when more energy is stored in the supply, creating a design conflict between overstress protection and sound quality.
Since the above methods to solve two conflicting design problems require the addition of extra components that increase amplifier cost and weight, a need exists for a new approach, thus the alternate solution below.